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Thread: Tap tone

  1. #1
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    Default Tap tone

    I don't usualy get involved with technical stuff like this..I just work to known dimensions..But I have noticed on almost all my soprano's that tapping on the bridge with my banana like finger before stringing up almost always gives the same tone note at around Middle "C" ..Could it be that this is the secret to building a good sounding uke..for instance if a uke had a taptone of "D" would it sound best tuned to "D" ??..also blowing across the soundhole flute style gives the same note.
    Last edited by Timbuck; 10-09-2016 at 12:30 AM.
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  2. #2
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    The soundhole will be the main mode or the helmholtz resonance. There is also a top mode, which may be the one that you get when you tap the bridge. I'm a little unsure on this stuff so I may have that wrong.
    Some makers of guitars will target a fixed note for the helmholtz. On classical guitars it's usually in the F to G# region. I don't target it myself but by default they do tend to end up in that region. Then again F to G# is four different notes and I've heard very good sounding instruments with any of those notes, so I doubt that one specific note is the secret formula.

  3. #3
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    There are at least two main resonances in our instruments, three if you have an active back. There is the top resonance which you can measure when you block the sound hole with something. The hole resonance of course shows up when you have it uncovered. Using a spectrum analyzer program and a cheap mic you can find these when tapping the top. The back resonance can be found with the sound hole blocked and tapping the back.

    These three are where the most of the sound comes from. Notes that do not fall on these frequencies still excite these resonances and use them to transform the top movement to sound. The top and back resonances work like a speaker diaphragm, with most of the lower bout area moving in and out. The sound hole acts as a port in a bass reflex speaker. Generally you would want these resonances not fall on scale frequencies as when on the scale tones the energy gets sucked out of the string very quickly. The note would be louder but not last very long. In guitars they are called wolf notes, originally the term comes from the violin family of instruments.

    The spectrum of a small guitar I built.



    As you can see the top and the hole resonances are the big dogs on the block, the other modes (ways the top vibrates) are lower down but contribute to the loudness of the instrument. The violin community has don a lot of work learning how instruments vibrate, classical guitar people have done some also. Since ukes are built similar to a classical I would think the same properties apply but are just scaled up in frequency. Not much done for steel sting guitars though.
    Last edited by printer2; 10-09-2016 at 03:00 PM.

  4. #4
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    Default

    I love this stuff. I don't really understand it, but it has to mean something. Not how to make a better sounding uke necessarily, but how the darn thing actually works. Fascinating... Hey thanks Printer2, but I have a question; what are the cross and long dipoles again? Also, is this a composite diagram of all the resonances? Shouldn't there be three diagrams showing each of the the three states? Top resonance with hole open, top resonance with hole closed and then back resonance (hole open)? Sorry, if this is a stupid question.

  5. #5
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    The air resonance is the top with sound hole open
    Koaloha Tenor 2016
    Hoffman ML ebony/red spruce tenor
    Mya-Moe redwood/walnut tenor
    Stansell Myrtle and POC flamenco baritone
    Hoffmann A style cedar maple tenor

  6. #6

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    Quote Originally Posted by sequoia View Post
    I love this stuff. I don't really understand it, but it has to mean something. Not how to make a better sounding uke necessarily, but how the darn thing actually works. Fascinating... Hey thanks Printer2, but I have a question; what are the cross and long dipoles again? Also, is this a composite diagram of all the resonances? Shouldn't there be three diagrams showing each of the the three states? Top resonance with hole open, top resonance with hole closed and then back resonance (hole open)? Sorry, if this is a stupid question.
    No, the graph is a frequency sweep using a constant amplitude to excite the instrument. The line is its amplitude response at each of the frequencies. The various resonances can be seen as peaks in the response.

  7. #7
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    I don't know. I've been reading a lot about this lately, and found people who swear that it's bunkum, others who are adamant that different parts of an instrument should be precisely tuned to specific frequencies, some who say it's about ensuring a range of overlapping responses, some who reckon it's useful for getting similar sounds out of dissimilar woods, etc. I have no idea, but I keep tapping and listening. It's surprised me how little wood needs to be removed from a top to deepen its tone.

    There must be enough instruments on this forum to get sufficient data to determine some interesting (perhaps even useful) results, if anybody is inclined to make a poll.

    For what it's worth, my favourite tenor gives a slightly sharp G when I tap the bridge. The tenor I'm building is at A#, but its currently got an open back and will get a bit more sanding before finishing.

  8. #8
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    Well I'm no wiser than I was a the start..Here is a demo of tap tone playing https://www.youtube.com/watch?v=UeD8xNeNqzM
    I tried this, and now my finger hurts.
    http://ukulele-innovation.tripod.com ebay i/d squarepeg_3000 Email timmsken@hotmail.com

    If you can believe that moving images and sound, can fly through empty space across the universe and be seen and heard on a box in your living room ?.. then you can believe in anything.

  9. #9
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    Quote Originally Posted by Dan Gleibitz View Post
    It's surprised me how little wood needs to be removed from a top to deepen its tone.
    Somogyi very clearly explains this by referring to the cube rule: the stiffness of a material is a cube function of its height and a linear function of its width. So, if you shave bracing or thin down tops, every Z you remove has a Z x Z x Z (Z cubed) effect on the stiffness at that point.

  10. #10

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    I understood from my engineering son, and I checked, that the strength of a beam (a brace is a beam after all) is proportional to it width and the square of the height, not the cube of the height.

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